(1) Field of the Invention
This invention relates generally to process and apparatus for regenerating wet spent carbon adsorbent. More particularly, the present invention is directed to a two stage procedure for regenerating wet spent carbon previously employed for purification of a liquid or gaseous stream.
(2) Description of the Prior Art
Carbon is well known for its properties of adsorption and/or absorption (commonly referred to as "sorption") and has long been used for the removal of contaminants from liquids and gases. In the area of decolorization and waste water treatment, carbon has found widespread utilization and efficacy.
Where the quantity of fluid being treated is relatively small, carbon is used until fully spent and then is discarded. On the other hand, in large scale industrial applications, the large quantities of carbon required mandates reuse of carbon for cost effectiveness purposes. Regeneration techniques involving solid sorbents, e.g., alumina, silica gel, molecular sieves and activated carbon have been developed and refined over the years.
The standard means of carbon regeneration process involves removal of water and adsorbate from the carbon to produce regenerated carbon, which can be reused. In removing the water and adsorbate from the carbon, the water is vaporized to moisture and the adsorbate is pyrolyzed to volatiles and fixed carbon. The fixed carbon is removed from the regenerated carbon by chemical reaction, normally with steam to form hydrogen and carbon monoxide. The regeneration off gas stream, which contains the organic volatiles, hydrogen, carbon monoxide, moisture, particulates, and other inorganic volatiles, is not normally suitable for emission to the environment. Normally the combustibles including the organics are combusted in an afterburner to carbon dioxide and water and the inorganics to the oxidized forms which are removed along with the particulates in a scrubber. The scrubber water must be treated accordingly and the clean gas is exhausted to the atmosphere.
In the fluid bed carbon regeneration process described in application Ser. No. 947,059, now U.S. Pat. No. 4,248,706, now allowed, by A. J. Repik, the external afterburner is eliminated and the heat is used within regeneration to improve the heat economy of the total regeneration process. In the Repik regeneration process the wet spent carbon is also dried, but the heat from incineration of the adsorbate is utilized in this case to drive the moisture from the carbon. In the second stage of a two stage process the adsorbate is volatilized and pyrolyzed, and the fixed carbon is reacted with steam to form hydrogen carbon monoxide and regenerated carbon. The term "fixed carbon" is intended to describe the residual carbonaceous product remaining on the carbon sorbent from the pyrolysis reaction of the adsorbed impurities.
Because of the efficiency of gas/solid contact in fluid beds, smaller reactors or less heat is required than in moving bed (multihearth or rotary) regenerators. In addition, the Repik regeneration process has improved economy over other fluid bed regenerators in that less fuel is required for regeneration because the adsorbate (volatiles, H.sub.2, and CO) are combusted in the incinerator section above the regenerator bed. Heat is radiated and conducted from the incineration zone to the regenerator bed to reduce the amount of supplemental fuel needed for carbon regeneration. The incinerated gas is tempered as needed and used to dry the spent carbon in a fluid bed dryer. The off gas from the dryer contains the particulates and oxidized form of inorganics, which may be removed as necessary in a scrubber. The clean gas is exhausted to the atmosphere.
The drying of the spent carbon is normally carried out at temperatures which may result in steam stripping of organics, depending on the adsorbate. If the adsorbate impurities which are volatilized in the dryer are not suitable for exhaustion to the atmosphere, then they must be removed from the gas stream. Many industrial situations would be expected to require an afterburner between the carbon regenerator and scrubber.
Although the Repik application teaches the incineration of off gases to liberate heat for heating the material being processed and the specific utilization of incinerated off gases for directly contacting wet, spent carbon prior to regeneration, there is no teaching of reincineration of the drying gases to completely remove volatile impurities which may have been added to the incineration zone gaseous effluent during drying the wet, spent carbon.
It is, therefore, an object of this invention to regenerate wet spent carbon sorbent, preferably obtained from a water purification system, in a manner such that activity of the carbon is not adversely affected by excessively elevated combustion temperatures at the loci of carbon treatment thereby enabling its subsequent reuse. Another object of this invention is to provide for recovery of heat values from the products of reactivation and to render the sorbed gases harmless prior to discharge to the atmosphere. A further object of the invention is to beneficially employ the recovered heat values from reactivation in a separate drying stage. Most importantly, the improvement of this invention allows this described recovered heat value's beneficial use to occur while maintaining the harmless nature of the sorbed gases, allowing their discharge to the atmosphere.